Internal combustion engines generally include an engine block housing one or more cylinders and a corresponding number of pistons which seal against the cylinder sidewalls during operation in order to extract power from the combustion cycle. The cylinders are typically cast as rough bores, and may subsequently be machined to produce a cylinder with a desired internal diameter or, alternately, lined with a cylinder sleeve having the desired internal diameter during the manufacture of the block. In either approach it is difficult to obtain a precise internal diameter for the cylinders, as the tools used to size and finish the cylinders or cylinder sleeves have positioning tolerances and are subject to wear over time. Consequently, the cylinders within a single engine block, as well as the cylinders within nominally identical engine blocks manufactured over time, tend to have actual internal diameters which vary from a nominal diameter within an accepted tolerance. For example, in an engine with 54 mm (2.1260 inch) cylinders, the tolerance for the internal diameter may be +0.014/−0.000 mm (+0.0005/−0.000 inch), and the individual cylinders may have actual internal diameters of 54.000-54.014 mm (2.1260 to 2.1265 inches).
The pistons, which must essentially conform to the internal diameter of the cylinders during operation, also tend to have actual diameters which vary from a nominal diameter within an accepted tolerance. For example, on modern manufacturing equipment pistons for 54 mm cylinders may be turned with a tolerance for the external diameter of +0.000/−0.014 mm (+0.0000/−0.0005 inch). However in high performance or low emission engines the tolerance for the proper piston-to-cylinder wall clearance, typically measured at a predetermined location along the piston skirt, may be substantially less than the manufacturing tolerances for the cylinder and piston components, e.g., ±0.0035 mm (±0.00014 inch), or about one fourth of the individual tolerances described above. Under these constraints, manufacturing a single piston part sized for a cylinder having the nominal internal diameter would be undesirable, since a piston that is undersized with respect to the cylinder will contribute to a reduction in peak compression and efficiency, as well as an increase in combustion gas blow-by, oil deterioration, and operating noise. On the other hand, manufacturing a single piston for an engine cylinder having a ‘typical’ internal diameter, e.g., a cylinder having the mean or median internal diameter of that permitted by the manufacturing tolerance, would be undesirable since a piston that is oversized with respect to a cylinder will contribute to an increased risk of piston seizure, as well as an increase in frictional losses, piston wear, and cylinder wear.
Current practice seeks to satisfy piston-to-cylinder wall clearance tolerances by matching individual cylinders with individual pistons. Each cylinder in an engine block is measured in a controlled environment and graded with a designation that indicates the degree of variation from the nominal cylinder diameter, e.g., following the above-described example, with a letter “A” to designate a cylinder with an actual internal diameter of 54.000-54.002 mm, a letter “D” to designate a cylinder with an actual internal diameter of 54.012-54.014 mm, and intermediate letters to designate intermediate internal diameters. The grade designations are typically marked, stamped, or otherwise provided on the engine near each cylinder so that an appropriately sized piston can be fitted into the cylinder during the manufacturing of the engine and any subsequent repairs. At the same time, original and aftermarket parts manufacturers will manufacture, measure, and grade a line of pistons having a range of actual diameters appropriate for the range of potential cylinder diameters, e.g., following the above-described example, a 54 mm “A” piston having an external diameter of 53.957-53.960 mm (2.1243 to 2.1244 inches), a 54 mm “D” piston having an external diameter of 53.969-53.972 mm (2.1248-2.1249 inches), etc., so that by matching the marked grades assembly workers can draw from stocks of appropriately sized pistons and end users can order appropriately sized piston kits. However this practice requires manufacturers and their distributors to maintain complex inventories of pistons for not only all of the models of engines within their target markets, but also for all of the grades of cylinder sizes established for each model of engine. Thus the practice of grading cylinders and pistons leads to increased administrative and capital expenses due to larger inventories, as well as a heightened potential for shortages of particular pistons due to the specialized nature of the inventory.
Therefore there is a need for a method of fitting a piston into the cylinder of an internal combustion engine that can overcome the limitations which have lead to the use of grading for achieving proper piston-to-cylinder wall clearances, and reduce or even eliminate the need to manufacture, measure, and inventory a well populated range of piston grades for the assembly and servicing of a particular model of engine.